The present invention relates to a new and improved construction of automatic displacement or movement measuring system such as employed, by way of example, at machine tools or theodolite devices for the continuous digital display of the position or setting of a component of such machine or device, which component can be forwardly and rearwardly displaced in linear motion or in rotation about an axis relative to an actual or virtual reference position to which there is assigned a zero display. Another possible field of application of such type measuring system is as an actual value transmitter in automatic control devices for positioning movable components, especially in devices of the type wherein signals are processed and displayed digitally.
Basically two types of automatic displacement or movement measuring devices are known to the art. In the first type, the position of the component is measured and eventually indicated in the form of a so-called absolute value which is independent of what has previously occured and is deduced according to a code from readings taken at a multiplicity of binary coded tracks of a scale carrier, for instance with photoelectric reading means. This absolute value of the position is in fact equivalent to the displacement of the component from a reference position. Examples of such type position-measuring devices have been disclosed in Swiss patent 374,207 and U.S. Pat. No. 2,979,710 (Toth). For measuring the position of a component carrying out a linear motion over a total length of 500 millimeters with a desired accuracy of 1:50000 it is necessary to accurately measure the absolute values of position in units of 0.01 millimeter, for which purpose there is required a 16-track scale. Similarly, for an angle-measuring device possessing approximately the same accuracy of 1:50000 at the most finely graduated code track supported at a rotatable disk of about 20 centimeters diameter, there is also required a 16-track scale for the resolution of the absolute angular values to be obtained, the most finely graduated code track then having graduation units of about 0.008 centesimal grade (periphery = 400 centesimal graduations). It should be readily apparent that such type absolute-value measuring devices are complicated to manufacture and are subject to functional disturbances because each local contamination of a graduation track can trigger an erroneous display and also because the most finely graduated code track cannot be read with certainty.
A second class or type of automatic displacement measuring devices have become known to the art under the general designation of "increment-counter systems". In such systems a stationary reading means cooperates with a single increment-bearing track, i.e. a single scale track uniformly divided with the desired fineness into graduation units of equal magnitude, the scale track being located at a scale carrier affixed to the component displaceable in linear or angular motion. The reading means is operatively associated with electrical circuits in order to properly count, with regard to the direction of motion, each traversing of a graduation unit of the increment scale in front of the reading means and at any instance in time to deliver as a measurement value the algebraic sum of the counted graduation units (plus and minus signs being assigned to each direction of motion, respectively). Under the precondition that counting occurs free of error, the algebraic sum of the counted graduation units then corresponds to the desired absolute value of position, assuming a starting position of the scale carrier has been defined and associated with a predetermined absolute value, preferably zero.
An exemplary embodiment of such type incremental measurement system has been disclosed in Swiss patent 499,091. There are provided scale-carrier disks each having a circular-shaped graduation track of 50,000 periods of graduation units, each of which consist of a pair of strips which are respectively opaque and transparent. Each strip possesses the width of one-half of a graduation unit. The graduation tracks are displaced relative to one another in order to possess a certain mutual eccentricity. This prior art increment-measuring device or system also comprises stationary photoelectric reading means, each embodying a respective illumination mechanism and, for instance, three photocells as well as electrical circuits for processing the photocell currents. The reading means are associated with further electrical circuits in order to properly count, with respect to the direction of motion, the moire fringes resulting from the common illumination and the cooperation of both graduation tracks. It is a property of such moire fringes that they form a pattern of light and dark fringes having a period much larger than the period of the pattern-generating graduation tracks, thus allowing the reading means to be of larger dimension. The three photocell currents of each reading means are combined into a three-phase current system from which there can be derived a multi-phase (for instance a two-phase) binary signal system which is periodic in graduation units of the graduation tracks, and the periods of which can be counted rapidly, sign-correct, and free of any disturbance in a counter.
Another embodiment of such type incremental measuring system has been disclosed in U.S. Pat. No. 3,591,841 (Heitmann et al.), wherein at least one increment-scale track supported at a movable scale carrier is associated firstly with a photoelectric reading means comprising a plurality of sensing fields and secondly with a stationary graduation carrier having unequally spaced respective opaque and transparent portions. The cooperation of the stationary graduation carrier with the plurality of sensing fields and the increment-scale track results in a coded output of the reading means delivering the displacement magnitude as well as the displacement direction of the increment scale. In this state-of-the-art system the absolute value of the scale-carrier position is evaluated by means of a plurality of additional binary code tracks delivering a coarse digital indication implementing the increment-scale reading. Such an arrangement requires a plurality of binary code-reading means in addition to at least one increment-reading means. Furthermore, the overall precision of the increment-reading cannot be enhanced by use of the moire technique, the latter being incompatible with the increment-scale reading system as disclosed; accordingly, the reading accuracy is limited to the order of magnitude of the grating accuracy of the finest increment-scale track.
According to other prior art incremental measuring system there is arranged at a movable scale carrier at least one increment-scale track and at least one auxiliary track having auxiliary marks, both tracks being disposed parallel or concentric, as the case may be. The counter state of an increment counter associated with the increment-scale track and its respective reading means is synchronized by the pulses generated by auxiliary reading means scanning the auxiliary marks, i.e. there is insured that after a displacement of the scale carrier through a path corresponding to a period interval of auxiliary marks the counter state of the increment counter, if necessary, is either forwardly or rearwardly stepped or reset to the correct counter state. Such prior art incremental measuring systems are disclosed in several publications.
For instance, in U.S. Pat. No. 3,122,735 (Townsend) there are disclosed several circular increment-scale tracks of which the one with the largest period interval bears a single auxiliary mark, the period interval of the latter track thus constituting one complete revolution of the scale carrier. This single auxiliary mark defines a reference position of the scale carrier which is associated with the value zero displayed at the counter.
In U.S. Pat. No. 3,544,800 (Elliott) there are taught two circular tracks, one constituting an increment-scale track associated with respective reading means comprising means for generating and reading a moire pattern, the other constituting an auxiliary scale track bearing a random arrangement of marks cooperating with respective auxiliary reading means comprising a mask, the latter bearing an arrangement of masking marks correlated to the random arrangement of marks on the auxiliary-scale track: thus there is a single reference position of the scale carrier in which a signal is generated at the auxiliary reading means to reset the counter to the value zero associated with this single reference position.
In U.S. Pat. No. 3,024,986 (Strianese et al.) there are taught three circular tracks, each associated with respective reading means, one track defining a first increment-scale track, another track defining an auxiliary scale track having a single auxiliary mark defining a single reference position of the scale carrier, and still another track defining a second increment-scale track, pitch of which is slightly different from the pitch of the first increment-scale track. Both increment-scale tracks cooperate with each other and with their respective reading means and the associated electrical circuits to generate a vernier effect (which can be considered equivalent to a moire effect), the resulting measurement thus being more accurate than a direct reading of a single increment-scale track.
In a particular system as disclosed in German patent publication 1,214,892 the starting point of the auxiliary track is correlated to the position of the scale carrier at the start of the measurement operation, so that such start position is taken as a reference position associated with the value zero at the increment counter.
The previously discussed type systems have been found to be particularly satisfactory in the case of highly accurate angle and length measurements. Yet, these systems are all associated with the drawback that in the event of a breakdown during operation there is lost the correct counter state and thus the absolute value of the magnitude to be measured. Accordingly, when the system is again placed into operation the movable component must be first brought to the reference position in order to permit the increment counter to be readjusted to the correct value, preferably zero. This is oftentimes not possible at all, for instance, in the manufacture of parts wherein the reference point or surface thereof gets lost as their manufacture proceeds or, for instance, in the case of weapons and other devices which only can be adjusted over a limited range of movement.
There have been proposed several solutions to eliminate this drawback by providing more than one reference position of the movable component, the counter state being set to the appropriate value at either reference position which may be used. According to an incremental measuring system as disclosed in Swiss patent 472,021 there is arranged at a movable scale carrier, parallel to the increment-scale track, a second track having equally spaced auxiliary marks whose intervals can be identified, i.e. these intervals can be objectively differentiated. For this purpose the second track comprises, on the one hand, wide graduations, the one edge of which serve as auxiliary marks, and, on the other hand, increment graduations, the number of which serves as an absolute-value indication of the associated auxiliary mark. The reading means associated with the second track is thus necessarily constructed such that individual increments can be read-out. With this system there is present the drawback that when a very high degree of fineness of the increment graduations is required on each track in order to attain a very high overall accuracy of the system, there is simultaneously required a very high resolution capability and a very high sensitivity of the reading means for the second track, since it is not possible to use a moire technique for counting the increment graduations between the wide marks of such second track. Such solution is therefore extremely sophisticated, and the limits which can be technologically attained remain considerably below the resolution capacity and sensitivity of a moire technique. The accuracy of the absolute-value indication is at most equal to the accuracy of the reading-out operation at the auxiliary mark track. With the aforementioned incremental measuring system there is indeed provided at the increment-scale track a means for generating and reading moire fringes, whereby it is possible to determine differential values with the high accuracy of the moire technique. Yet, the absolute-value indication is limited to the accuracy of the reading of the increment graduations on the second track, so that there is destroyed the usefulness of the entire means for generating and reading moire fringes and the overall accuracy of the system is restricted to the accuracy of the absolute-value determination.
U.S. Pat. No. 3,663,803 (Mohan et al.) teaches a system for unambiguously reading a plurality of marks located on a scale track of a movable scale carrier. The resulting mark signal is enhanced by matching opaque and transmitting (or light and dark) portions of the marks with respective portions of photoelectric reading means. The system is devised to read the marks while the scale carrier is moving, i.e. the waveform and frequency of the mark signal is analyzed to identify the mark. It is not possible to sharply define the position of any mark on the scale track, because the reading of the mark necessitates the displacement of the scale carrier to a certain extent, a given mark being identified only after it has wholly traversed in front of the reading means.
In order to solve the problem of enhancing the accuracy when reading the position of absolute-value marks, U.S. Pat. No. 3,749,926 (Hertrich) teaches a movable scale carrier having a scale track with a scale combining absolute-value and increment marks. The scale track comprises two sub-tracks associated with respective photoelectric reading means each generating a sub-signal, with masks and with electrical circuits for combining the two resulting sub-signals into a single mark signal. In each sub-track there are provided opaque and transparent portions in an arrangement such that the sub-signals have a predetermined phase-relationship with respect to the motion of the scale carrier. The combination of the two sub-signals results in a non-zero mark signal on the one side of the mark, whereas on the other side of the mark there are zeroes of the mark signal which appear with the periodicity of the increment scale. In the vicinity of the absolute-value mark location there is provided a large change in phase of the sub-signals with respect to each other, in order to obtain a mark signal having zeroes at scale carrier locations as sharply defined as possible. In this system, however, there is not possible any identification of one selected absolute-value mark among several such marks, so that only a single mark may be used, for instance to identify a single reference or starting position of the scale carrier, i.e. of the movable component. Furthermore, the starting position is defined as the location where there is obtained one selected zero of the mark signal, preferably, but non necessarily, the first such location. Such system thus may be used only for one predetermined direction of movement of the scale carrier, which is the one direction for which no zero of the mark signal does occur before the absolute-value mark has traversed in front of the reading means and for which the mark signal is periodically zero after the absolute-value mark has traversed. Such a system is therefore neither usuable in connection with a plurality of absolute marks, nor for measurements at components movable in either possible direction of movement.